let us easily override PHPUnit version

The new --with-phpunitdir let us specify a git checkout of PHPUnit. That
will ease regression testing of the upstream new version. Usage is
straightforward:

Checkout PHPUnit from git as instructed on:
https://github.com/sebastianbergmann/phpunit#readme

Invoke our test suite with:
 $ cd maintenance/tests
 $ php phpunit.php --with-phpunitdir /path/you/just/checked/out

Cherry-picked from 3b9a3678b35ba6c2c4158d230393161a37f21e86

Change-Id: I8a9af0deac4dd74e3c8bde73535555c54ac83766